1. Field of the Invention
This invention relates to an ink jet recording apparatus of the continuous jet type, and, more particularly, to arm ink jet recording apparatus of the continuous jet type wherein the number of drops of ink to be placed into a picture clement is variably controlled to adjust the diameter of a recorded dot of ink in order to achieve a representation with the density of ink.
2. Description of the Related Art
link jet recording apparatus of the continuous jet type are already known and disclosed, for example, in U.S. Pat. No. 4,620,196 or Japanese Patent Laid-Open Application No. 62-225362 wherein the number of drops of ink to be placed into a picture element is variably controlled to adjust the diameter of a recorded dot of ink in order to achieve a representation with the density of ink using the technique of ink jet recording of the continuous jet type.
As shown in FIG. 2, there is shown general construction of an exemplary one of conventional ink jet recording apparatus of the continuous jet type employing a rotary drum. The continuous jet type ink jet recording apparatus shown includes, as principal components thereof, a nozzle 1 to which ink under pressure is supplied by known means not shown, an ink electrode 2 for connecting the potential of the ink in the nozzle 1 to the ground potential level, a piezoelectric vibrating element 3 mounted on the nozzle 1, an oscillator OSC for generating a disintegrating frequency signal f.sub.d having a fixed disintegrating frequency f.sub.d (in the following description, a same reference character is applied to both of a signal and a frequency), a vibrating element driver CD for amplifying the disintegrating frequency signal f.sub.d from the oscillator OSC to drive the piezoelectric vibrating element 3 and synchronously disintegrate a jet of the ink (which may be hereinafter referred to as jet or ink jet), a controlling electrode 4 having a circular opening or a slit-like opening coaxial with the nozzle 1 for receiving a charging controlling signal to control charging of the ink jet in accordance with image data, a grounding electrode 5 disposed in front of the controlling electrode 4 and grounded itself, a knife edge 6 mounted on the grounding electrode 5, a deflecting high voltage dc power source (hereinafter referred to simply as deflecting power source) 7, a deflecting electrode 8 connected to the deflecting power source 7 for cooperating with the grounding electrode 5 to produce therebetween an intense electric field perpendicular to an ink jet flying axis to deflect a charged ink drop to the grounding electrode 5 side, a line buffer LB for storing therein picture element data for one rotation of a rotary drum DR, a pulse width modulator PWM for modulating picture element data read out from the line buffer LB in synchronism with an encoder clock signal f.sub.E from a shaft encoder SE coupled to a shaft of the rotary drum DR into a width of a pulse in synchronism with the encoder clock signal f.sub.E and the disintegrating frequency signal f.sub.d outputted from the oscillator OSC, a high voltage switch HVS for converting a charging controlling signal outputted from the pulse width modulator PWM into a high voltage, a frequency divider DV for frequency dividing the disintegrating frequency signal f.sub.d outputted from the oscillator OSC to 1/n (n is a positive integral number), and a drum motor controlling circuit DMC for receiving the output f.sub.d /n of the frequency divider DV as an instruction input and the encoder clock signal f.sub.E from the shaft encoder SE as a feedback input and controlling the rotation of a drum motor DRM coupled to the shaft of the rotary drum DR so that the received signals f.sub.d /n and f.sub.E may satisfy the relationship f.sub.E =f.sub.d /n. A record medium RM is wrapped around the rotary drum DR.
In the continuous jet type ink jet recording apparatus of the construction described above, a picture element is recorded on the record medium RM in synchronism with the encoder clock signal f.sub.E. Meanwhile, an ink drop is formed in synchronism with the disintegrating frequency signal f.sub.d outputted from the oscillator OSC. Further, the drum motor controlling circuit DMC controls the rotation of the drum motor DRM so that the relationship f.sub.E =fd/n may be satisfied. Accordingly, at the greatest a total number n of ink drops which depends upon the frequency dividing ratio n of the frequency divider DV are allotted to one picture element (in the following description of the conventional continuous jet type ink jet recording apparatus, the value n will be hereinafter referred to as maximum ink drop number per picture element). Each of picture element data stored in the line buffer LB represents the number of those of maximum n ink drops per picture element which are to be used for recording of the picture element, and the pulse width modulator PWM operates in accordance with such picture element data so that a shaded image, that is, an image having gradation in density, is recorded on the record medium RM. In this instance, a half tone representation of n gradations can be obtained. Thus, with the conventional continuous jet type ink jet recording apparatus, the maximum ink drop number n per picture element coincides with the gradation number n.
By the way, the maximum ink drop number n per picture element with which an appropriate image density is obtained (that is, an appropriate adhered ink amount) varies to a great extent depending upon the picture element density and the ink absorbing capacity of the record medium. For example, when a light transmitting original such as a film for an OHP (overhead projector) is to be produced, the amount of ink equal to almost twice that of ink used for a light reflecting print of ordinary paper or the like must be adhered to the film.
The conventional continuous jet type ink jet recording apparatus described above is disadvantageous in that it can record only on very limited record media, such as ordinary paper, since the maximum ink drop number n per picture element is a fixed value.
It is further disadvantageous in that it is very low in flexibility in that, when the maximum ink drop number n per picture element is to be changed, a modification to the hardware or firmware is required.